Eddy current testing is one way to test metal parts for cracks and other flaws. U.S. Pat. No. 5,130,651 discusses the use of eddy current probes for that purpose. Generally speaking, eddy current testing involves a process by which the metal part is exposed to a magnetic field, producing electric currents in the part. The path taken by these currents is disturbed by a flaw, altering the overall field in the area of the flaw. By comparing the field at different locations to a known field for a flawless area, the flaw can be identified.
In a gas turbine engine, compressor, turbine and fan blades are removably attached to alloy disks in so-called "broaches", which are specially machined slots along the outer edge of the disk that receive an identically profiled blade base. Cracks can be present in the curved regions in the broaches, but the eddy current patterns produced by the broaches can mask the crack when conventional eddy current testing techniques are used.